Figure 5.

Fibre cell denucleation in rodent lenses. (a) Programmed degradation of nuclei occurs in cortical lens fibre cells of rats raised under normal conditions. (b) Chromatin breakdown is inhibited in rats fed a tryptophan-deficient diet. (c) Transient reintroduction of tryptophan causes chromatin breakdown in fibres that underwent denucleation during the period tryptophan was present. (d) Electron micrograph of a nucleus from the cortex of a tryptophan-deficient lens. Note the segregation of chromatin/DNA (electron dense) from other nuclear constituents. Inset shows a light micrograph of the same nucleus stained with Hoechst 33258 dye to visualize DNA. (e) At higher magnification, small (approx. 10 nm) particles (putative proteasomes) are visible in the matrix of disintegrating nuclei tryptophan-deficient rats. (f) Similar condensed chromatin/DNA is also found in normal rat lenses and in young human lenses. (g) Denucleation also occurs in the cortex of wild-type mouse lenses but is blocked in lenses from DNase-IIβ-null mice (h). Ep, epithelium; OFZ, organelle-free zone.